A fault event of a circuit arrangement for operating a discharge lamp with preheatable electrodes generally occurs directly at the lamp generator, for example when the lamp is removed, in the case of a lamp which does not start or at EoL (End of Life). Once a fault event has been detected, the lamp generator then remains switched off until the normal state is reproduced. In this case, one or more electrodes of the discharge lamp are generally used for the purpose of obtaining information on the proper connection of the discharge lamp to the circuit arrangement. If the discharge lamp is replaced, a circuit is interrupted which has maintained this switch-off state. This provides the possibility for the discharge lamp to be restarted.
The switching unit provided in a generic circuit arrangement for implementing power factor correction is often realized by means of a standard PFC (Power Factor Correction) IC (Integrated Circuit), for example using the L6562 by STM. In order to avoid intermittent operation of the switching unit for power factor correction when the lamp generator is switched off, measures therefore need to be taken to ensure that this switching unit remains switched off until the normal state is reproduced even when its activity is resumed.
One solution which could consist in routing the startup circuit of the switching unit for power factor correction via at least one lamp filament is nevertheless an option since otherwise the so-called active “pull down” of the power switch driven by the switching unit for power factor correction, which power switch is generally a MOSFET, could not be ensured under all operating conditions and the risk of destruction of this power switch would arise.
In the prior art, i.e. in a generic circuit arrangement, the integrated circuit measures the input voltage and the output voltage. If the values are in a predetermined value range, a start sequence is triggered. Off load, i.e. when a lamp is not inserted, the output voltage UA is in this case very high. During intermittent operation, voltages of up to 480 V were measured across the lamp terminals. The circuit arrangement therefore falls under a safety class which entails complicated precautions for preventing accidents. The event of a predeterminable threshold value for the output voltage being exceeded is established by the integrated circuit, which then switches off. As a result, the output voltage is reduced again, which in turn is established by the integrated circuit and results in a renewed start sequence being triggered. This procedure is repeated although no lamp has been inserted, and this results in heavy loading of the components involved.